1. Field of the Invention
The present invention relates to a transmission electron microscope for correcting aberrations in electron beams transmitted through a specimen.
2. Description of Related Art
In recent years, transmission electron microscopes have tended to have higher resolutions. The resolution of a transmission electron microscope is chiefly limited by aberration in the electrical lens system that focuses the electron beam onto a fluorescent screen or the like. Accordingly, in a known procedure, the resolution is improved by measuring aberration in the lens system and correcting the aberration in the lens system based on the data obtained by the measurement (see, for example, Japanese Patent Laid-Open No. H7-220669 (page 1, FIG. 1)).
In the measurement of the aberration, the electron beam is made to hit an amorphous reference specimen at various angles of irradiation. A transmission image formed on the fluorescent screen at each angle of irradiation is recorded as digital information about a transmission image. Since the information about the transmission image contains information about the aberration in the lens, quantitative information about the aberration is gained by image analysis, and aberration coefficients for electrically correcting the lens system are found from the information about the aberration. Fumio Hosokawa, “Automatic Tuning of TEM Using Diffractograms”, Journal of the Japanese Society of Microscopy, 2001, Vol. 36, No. 2, p. 115.
However, with the background art described above, much labor and time are required to gain information about transmission images. That is, in order to derive quantitative information about aberration by image analysis, it is necessary that information about many transmission images be gained by varying the angle of irradiation at which an electron beam is repeatedly made to hit a reference specimen.
Especially, when accurate aberration information or information about higher-order aberrations is derived, it is necessary that the angle of irradiation of the electron beam be varied more finely and that information about more transmission images be image-processed. In this case, still more labor and time are required to gain information about transmission images.
Consequently, it is important to realize a transmission electron microscope which can complete acquisition of transmission image information providing a basis of aberration information in a short time and which quickly corrects aberration.